The duckweeds are the sole members of the monocotyledonous family, Lemnaceae. The four genera and 34 species are all small, free-floating, fresh-water plants whose geographical range spans the entire globe. Landolt, Biosystematic Investigation on the Family of Duckweeds: The family of Lemnaceae—A Monograph Study. Geobatanischen Institut ETH, Stiftung Rubel, Zurich (1986). Although the most morphologically reduced plants known, most duckweed species have all the tissues and organs of much larger plants, including roots, stems, flowers, seeds and fronds. Duckweed species have been studied extensively and a substantial literature exists detailing their ecology, systematics, life-cycle, metabolism, disease and pest susceptibility, their reproductive biology, genetic structure, and cell biology. Hillman, Bot. Review 27, 221 (1961); Landolt, Biosystematic Investigation on the Family of Duckweeds: The family of Lemnaceae—A Monograph Study. Geobatanischen Institut ETH, Stiftung Rubel, Zurich (1986).
The growth habit of the duckweeds is ideal for microbial culturing methods. The plant rapidly proliferates through vegetative budding of new fronds, in a macroscopic manner analogous to asexual propagation in yeast. Duckweed proliferates by vegetative budding from meristematic cells. The meristematic region is small and is found on the ventral surface of the frond. Meristematic cells lie in two pockets, one on each side of the frond midvein. The small midvein region is also the site from which the root originates and the stem arises that connects each frond to its mother frond. The meristematic pocket is protected by a tissue flap. Fronds bud alternately from these pockets. Doubling times vary by species and are as short as 20–24 hours. Landolt, Ber. Schweiz. Bot. Ges. 67, 271 (1957); Chang et al., Bull. Inst. Chem. Acad. Sin. 24, 19 (1977); Datko and Mudd., Plant Physiol. 65,16 (1980); Venkataraman et al., Z. Pflanzenphysiol. 62, 316 (1970).
Intensive culture of duckweed results in the highest rates of biomass accumulation per unit time (Landolt and Kandeler, The family of Lemnaceae—A Monographic Study. Vol. 2: Phytochemistry, Physiology, Application, Bibliography, Veroffentlichungen des Geobotanischen Institutes ETH, Stiftung Rubel, Zurich (1987)), with dry weight accumulation ranging from 6–10% of fresh weight (Tillberg et al., Physiol. Plant. 46, 5 (1979); Landolt, Ber. Schweiz. Bot. Ges. 67, 271 (1957); Stomp, unpublished data). Protein content of a number of duckweed species grown under varying conditions has been reported to range from 15–45% dry weight (Chang et al, Bull. Inst. Chem. Acad. Sin. 24, 19 (1977); Chang and Chui, Z. Pflanzenphysiol. 89, 91 (1978); Porath et al., Aquatic Botany 7, 272 (1979); Appenroth et al., Biochem. Physiol. Pflanz. 177, 251 (1982)). Using these values, the level of protein production per liter of medium in duckweed is on the same order of magnitude as yeast gene expression systems. Prior to now, the systematic optimization of medium components and culturing conditions for maximal growth and protein content for specific duckweed strains has not been done.
Sexual reproduction in duckweed is controlled by medium components and culturing conditions, including photoperiod and culture density. Flower induction is a routine laboratory procedure with some species. Plants normally self-pollinate and selfing can be accomplished in the laboratory by gently shaking cultures. By this method, inbred lines of Lemna gibba have been developed. Spontaneous mutations have been identified (Slovin and Cohen, Plant Physiol. 86, 522 (1988)), and chemical and gamma ray mutagenesis (using EMS or NMU) have been used to produce mutants with defined characteristics. Outcrossing of L. gibba is tedious but can be done by controlled, hand pollination. The genome size of the duckweeds varies from 0.25–1.63 pg DNA/2C with chromosome counts ranging from 20 to 80 and averaging about 40 across the Lemnaceae (Landolt, Biosystematic Investigation on the Family of Duckweeds: The family of Lemnaceae—A Monograph Study. Geobatanischen Institut ETH, Stiftung Rubel, Zurich (1986)). Ploidy levels are estimated to range from 2–12 C. Id. Genetic diversity within the Lemnaceae has been investigated using secondary products, isozymes, and DNA sequences. McClure and Alston, Nature 4916, 311 (1964); McClure and Alston, Amer. J. Bot. 53, 849 (1966); Vasseur et al., Pl. Syst. Evol. 177, 139 (1991); Crawford and Landolt, Syst. Bot. 10, 389 (1993).
Accordingly, the characteristics described above make duckweed an ideal choice to develop as an efficient, plant-based, gene expression system.